The present invention is of advantage in the automotive field, where it is known to use a UEGO sensor, located along the exhaust duct of an internal combustion engine, to obtain information concerning the composition of the exhaust gases.
The UEGO sensor has two electrolytic cells responding to oxygen ions, and a diffusion chamber interposed between the actual cells and able to receive part of the combustion gases at the outlet from the engine. In addition, the UEGO sensor provides for the use of a test arrangement which is connected to the sensor itself by means of a connector, and is able to pilot, that is control, electrically one of two electrolytic cells to ensure a control action on the sensor. In particular the control means is designed to originate an oxygen ion drainage mechanism from the diffusion chamber to atmosphere and vice versa, in order to obtain within the chamber an exhaust gas composition equal to that which would be obtained in the event of the air/petrol ratio of the mixture fed to the engine becoming stoichiometric, i.e. equal to 14.57.
The intensity of the test action, that is to say the intensity of the control current required to maintain the stoichiometric level within the diffusion chamber, is the information on the basis whereof the test arrangement generates a Vout signal representative of the composition of the exhaust gases proceeding from the engine.
At present the aforesaid control arrangement, before installation on the vehicle, needs to be connected to a compensation resistance able to compensate eventual spreads of variations control current to ensure that the Vout signal is truly representative of the composition of engine exhaust gases. In effect, in the absence of a compensation resistance, the Vout signal would not be effectively representative of the percentage of oxygen in the exhaust gases owing to the swing in control current from the theoretical value due to inevitable losses in the control arrangement. The said compensation resistance, of which the nominal value is indicated by the sensor manufacturer for production purposes, is inserted between two connections of the said connector, and cooperating with the control arrangement, intervenes actively in the generation of the Vout signal recovering the losses in control current.
However, the compensation resistance, in addition to representing an additional cost and involving the installation operations, is subjected to strong thermal stress during the operation of the engine in that it is located in the engine compartment. Consequently, on varying the temperature of the engine it occurs that the compensation resistance assumes differing values in relation to the nominal value which ensures a correct compensation. This involves the control current spreads not being compensated adequately and the Vout signal not being effectively indicative of the true oxygen percentage in the exhaust gases and, consequently, of the effective rating of the mixture.